Integrated high speed modulator for grating-outcoupled surface emitting lasers
A single-mode grating-outcoupled surface emitting (GSE) semiconductor laser architecture is provided. This architecture enables high speed modulation of the GSE laser, which is accomplished by only varying the relative phase of counter propagating waves in the outcoupler grating region of the lasing cavity.
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1. Technical Field
The present invention is directed generally toward laser diodes, and more particularly to a grating-outcoupled surface emitting laser with an integrated high speed modulation.
2. Description of the Related Art
Transmission of light through waveguides has been pursued for many types of communications applications. Light signals offer many potential advantages over electronic signals. Light sources are commonly-created from semiconductor devices, and include semiconductor devices such as light emitting diodes (LED) and laser diodes (LD).
Optical fiber is the most commonly used transmission medium for light signals. A single fiber is capable of carrying several different modulated signals within it at one time. For instance, wavelength division multiplexing divides the used bandwidth of the fiber into different channels, each channel containing a small range of wavelengths, and thus transmits several different wavelengths (or signals) of light at once. Using such a system requires sources for the different wavelengths. More wavelengths on the fiber require more sources to be coupled to the fiber.
Efficient coupling of light into a fiber is simplified if the laser beam has a cross sectional profile that matches the profile of the fiber mode(s). Efficient use of light for communications requires that the light have high temporal coherence. Efficient coupling of light to monomode guides requires spatial coherence. Spatial coherence requires the laser to operate in a single lateral and transverse mode. Temporal coherence requires the laser to operate in a single longitudinal mode and implies a very narrow bandwidth, or range of wavelengths.
The most coherent semiconductor lasers use resonators based on grating feedback rather than Fabry-Perot resonators with reflective end facets. Distributed feedback (DFB) lasers use a Bragg reflective grating covering the entire pumped length of the laser. An alternative to DFB lasers is the use of distributed Bragg reflectors (DBRs) located outside the pumped region.
The Grating-Outcoupled Surface-Emitting (GSE) laser (described in commonly assigned U.S. patent application Ser. Nos. 09/844,484 and 09/845,029, both of which are hereby incorporated by reference), is an essentially planar structure that provides out-of-plane optical emission. The GSE laser has a built in horizontal waveguide that allows on-wafer or on-chip routing and control of light along with emission from the surface of the wafer or chip. In contrast, the light from vertical cavity surface-emitting lasers (VCSELs) is directed normal to the wafer or chip surface and cannot easily be routed within the wafer or chip. The epitaxial structure of a VCSEL is very thick and therefore costly and time consuming to grow, compared to the relatively thin layers making up an edge-emitting (EE) or GSE laser. While EE lasers have a horizontal waveguide and can route light within a wafer or chip, at least one terminating edge (cleaved or etched) is required to access or connect the on-chip light to the outside world. Thus EE lasers are inherently edge-bound (and hence not fully integrable), while VCSELs have incompatibility due to their very special epitaxy requirements.
When the laser is turned on and begins emitting light, photons are introduced into the lasing cavity by applying a current to the lasing cavity. The amount of light, or power, supplied by the laser must ramp up as the density of photons increases in the lasing cavity. Similarly, as the laser is turned off, the amount of power supplied by the laser decreases as the density of photons in the lasing cavity decreases. This process of turning the laser on and off takes time and affects the speed of communication for the laser.
SUMMARY OF THE INVENTIONThe present invention recognizes the disadvantages of the prior art and provides a single-mode grating-outcoupled surface emitting (GSE) semiconductor laser architecture. This architecture enables high speed modulation of the GSE laser, which is accomplished by only varying the relative phase of counter propagating waves in the outcoupler grating region of the lasing cavity.
BRIEF DESCRIPTION OF THE DRAWINGSThe novel features believed characteristic of the invention are set forth in the appended claims. The invention itself however, as well as a preferred mode of use, further objects and advantages thereof, will best be understood by reference to the following detailed description of an illustrative embodiment when read in conjunction with the accompanying drawings, wherein:
The description of the preferred embodiment of the present invention has been presented for purposes of illustration and description, but is not intended to be exhaustive or limited to the invention in the form disclosed. Many modifications and variations will be apparent to those of ordinary skill in the art. The embodiment was chosen and described in order to best explain the principles of the invention the practical application to enable others of ordinary skill in the art to understand the invention for various embodiments with various modifications as are suited to the particular use contemplated.
The present invention provides a single-mode grating-outcoupled surface emitting (GSE) semiconductor laser architecture. This architecture enables high speed modulation of the GSE laser, which is accomplished by only varying the relative phase of the counter propagating waves in the outcoupler grating region of the lasing cavity.
The lasing section consists of a gain region defined by an active ridge that is terminated at each end by a narrow-band, first-order, shallow, distributed Bragg reflector (DBR) 102, 104. DBR 102 is highly reflective (approximately 100%, for example) to only a single Fabry Perot wavelength at one end of the ridge. DBR 104 may be approximately 30% reflective, for example, to the same wavelength at the opposite end. In the lasing section the lasing action takes place between the two DBRs 102, 104 as a standing wave, which results from repeated interference of two counter propagating waves of identical frequency being formed inside the lasing cavity.
The second-order outcoupler gratings 112 are placed between the 30 percent reflective distributed Bragg reflector 104 and the phase section of the GSE device. On-resonance these second-order gratings 112 provide wavelength selective feedback and equally convert the incident power into output radiation in the air as well as the substrate. The period of the second-order grating 112 can be positively or negatively detuned from the DBR selected optical wavelength at which the laser operates. Detuning refers to how much the outcoupler wavelength deviates from the DBR wavelength. The main reason for detuning the second-order grating towards shorter or longer wavelengths is to obtain kink-free operation of the GSE laser. As shown in
The phase section of the GSE laser device in
First the laser section ridge is biased to a fixed current, ILASER, above the lasing section threshold. The laser current, ILASER, is applied to ridge 122 through contact pad 132. Next the laser is modulated by varying the current, IPHASE, of the phase section ridge 124 between two values that are both below the phase section threshold current. The phase current, IPHASE, is applied to ridge 124 through contact pad 134. Applying current to the phase section causes the effective index of the phase section to change by a process known as “gain induced index depression” in the active region (or quantum wells). As the relative phase between the counter propagating waves incident into both ends of the outcoupler region varies, the relative position of the standing wave changes with respect to the outcoupler grating teeth. This standing wave forms due to the repeated interference of the counter propagating waves. As the standing wave moves from an in-phase to an out-of-phase position with respect to the grating teeth, the outcoupled power peaks or nulls respectively (if the outcoupling grating is on resonance).
As shown in
The relative position of the standing wave formed in the outcoupler grating is difficult to control precisely. The two extreme cases of resonant standing-wave mode positions in relation to the second-order outcoupler grating teeth are shown in
From the plot shown in
The lasing action takes place between the two DBRs 702, 704. A standing wave, which results from repeated interference of two counter propagating waves of identical frequency, is formed inside the lasing cavity 722 by applying the laser current, ILASER, to ridge 722 through contact pad 732.
The second-order grating 712 outcouples the incident radiation into the air as well as into the substrate. The phase section of this GSE laser device also consists of a ridge region that excites the outcoupler end via DBR 704 opposite to DBR 702, both with the same grating period.
This GSE device is modulated as follows. First, the lasing section ridge is biased to a fixed current, ILASER (Ibias), above the lasing section threshold. The laser current, ILASER, is applied to ridge 722 through contact pad 732. Next, the laser is modulated by varying the current, IPHASE, of the phase section ridge 724 between two current correspond to in-phase and out-of-phase operation in the outcoupler grating. The phase current, IPHASE, is applied to ridge 724 through contact pad 734. As the relative phase between the counter propagating waves 15 varies inside the outcoupler region, the relative position of the standing wave changes with respect to the outcoupler grating teeth. This standing wave forms due to the repeated interference of the counter propagating wave. As the standing wave moves from an in-phase to an out-of-phase position with respect to the grating teeth, the outcoupled power peaks or nulls respectively.
The second-order outcoupler grating 912 is placed between the reflective distributed Bragg reflectors 902 and 904. This second-order grating 912 converts the incident power into output radiation into the air as well as into the substrate.
The laser is modulated by varying the phase currents IPHASE-1 and IPHASE-2 applied to ridges 942, 944. The phase current IPHASE-1 may be used, for example to adjust the phase to optimize power. The phase current IPHASE-2 may be used, for example to adjust the phase to turn the laser on and off by making the standing wave symmetric or antisymmetric with respect to the outcoupler gratings. As the relative phase between the counter propagating waves varies inside the outcoupler region, the relative position of the standing wave changes with respect to the outcoupler grating teeth. As the standing wave moves from an in-phase to an out-of-phase position with respect to the grating teeth, the outcoupled power peaks or nulls respectively.
Phase Shift Modulation
The sections above describe intensity modulation. A phase shift modulated output, instead of an intensity modulated output, may be obtained from each of the devices described above if the outcoupler grating is detuned approximately −15 nm or approximately +18 nm. In this case, the outcoupled intensity of the GSE laser does not change with current to the phase shift modulator (see
Claims
1. A method for high speed intensity modulation in a grating-outcoupled surface emitting laser device, the method comprising:
- applying a laser current to a first section in the grating-outcoupled surface emitting laser device to produce a standing wave that is substantially symmetric with respect to outcoupler grating teeth of the grating-outcoupled surface emitting laser device; and
- applying a phase current to a second section in the grating-outcoupled surface emitting laser device to vary the phase of counter-propagating waves between two values such that the counter-propagating waves alternate between in-phase and out-of-phase, the standing wave moves between a symmetric position and an antisymmetric position, and outcoupled light intensity is varied from a peak value to a null value.
2. The method of claim 1, wherein the outcoupler grating is operated on resonance.
3. The method of claim 1, further comprising:
- applying a power optimization current to a power optimization section in the grating-outcoupled surface emitting laser device to optimize power of outcoupled light.
4. The method of claim 1, wherein the phase of the counter-propagating waves is varied between in-phase and 180 degrees out-of-phase.
5. A method for high speed phase modulation in a grating-outcoupled surface emitting laser device, the method comprising:
- applying a laser current to a first section in the grating-outcoupled surface emitting laser device to produce a standing wave that is substantially symmetric with respect to outcoupler grating teeth of the grating-outcoupled surface emitting laser device, wherein the outcoupler grating teeth are at an off-resonance position that is chosen to be a point where a fraction of outcoupled light is independent of phase; and
- applying a phase current to a second section in the grating-outcoupled surface emitting laser device to adjust the phase of outcoupled light.
6. The method of claim 5, wherein the off-resonance position is a lasing wavelength of the grating-outcoupled surface emitting laser device minus approximately 15 nm.
7. The method of claim 5, wherein the off-resonance position is a lasing wavelength of the grating-outcoupled surface emitting laser device plus approximately 18 nm.
8. A grating-outcoupled surface emitting laser device comprising:
- a built-in horizontal waveguide;
- a second order grating within the horizontal waveguide, wherein the second order grating outcouples light;
- a laser section within the horizontal waveguide, wherein a bias current is applied to the laser section to generate a standing wave that is substantially symmetric with respect to outcoupler grating teeth of the second order grating; and
- a phase section within the horizontal waveguide, wherein a phase current is applied to the 2 0 phase section ridge and wherein the phase current is varied between two values such that the counter-propagating waves alternate between in-phase and out-of-phase, the standing wave moves between a symmetric position and an antisymmetric position, and outcoupled light intensity is varied from a peak value to a null value.
9. The grating-outcoupled surface emitting laser device of claim 8, wherein the laser section includes:
- a first highly reflective reflector at a first end of the horizontal waveguide;
- a partially reflective reflector; and
- a laser section ridge positioned between the first highly reflective reflector and the partially reflective reflector, wherein the bias current is applied to the laser section ridge.
10. The grating-outcoupled surface emitting laser device of claim 9, wherein the first highly reflective reflector is substantially 100% reflective.
11. The grating-outcoupled surface emitting laser device of claim 9, wherein the first highly reflective reflector is a distributed Bragg reflector.
12. The grating-outcoupled surface emitting laser device of claim 9, wherein the partially reflective reflector is substantially 30% reflective.
13. The grating-outcoupled surface emitting laser device of claim 9, wherein the phase section includes:
- a second highly reflective reflector at a second end of the horizontal waveguide; and
- a phase section ridge positioned between the partially reflective reflector and the second highly reflective reflector, wherein the phase current is applied to the phase section ridge.
14. The grating-outcoupled surface emitting laser device of claim 12, wherein the second highly reflective reflector is substantially 100% reflective.
15. The grating-outcoupled surface emitting laser device of claim 12, wherein the second highly reflective reflector is a distributed Bragg reflector.
16. A grating-outcoupled surface emitting laser device comprising:
- a built-in horizontal waveguide;
- a second order grating within the horizontal waveguide, wherein the second order grating outcouples light;
- at least a first laser ridge within the horizontal waveguide, wherein a first bias current is applied to the first laser ridge to generate a standing wave that is substantially symmetric with respect to outcoupler grating teeth of the second order grating;
- a first phase ridge within the horizontal waveguide, wherein a first phase current is applied to the phase section ridge to optimize peak power of outcoupled light; and
- a second phase ridge within the horizontal waveguide, wherein a second phase current is applied to the second phase ridge and wherein the second phase current is varied between two values such that the counter-propagating waves alternate between in-phase and out-of-phase, the standing wave moves between a symmetric position and an antisymmetric position, and outcoupled light intensity is varied from a peak value to a null value.
17. The grating-outcoupled surface emitting laser device of claim 16, further comprising:
- a first highly reflective reflector at a first end of the horizontal waveguide; and
- a second highly reflective reflector at a second end of the horizontal waveguide.
18. The grating-outcoupled surface emitting laser device of claim 17, wherein the first highly reflective reflector and the second highly reflective reflector is substantially 100% reflective.
19. The grating-outcoupled surface emitting laser device of claim 17, wherein the first highly reflective reflector and the second highly reflective reflector is a distributed Bragg reflector.
Type: Application
Filed: Feb 8, 2005
Publication Date: Aug 10, 2006
Patent Grant number: 7313158
Applicant: Photodigm, Inc. (Richardson, TX)
Inventors: Gary Evans (Plano, TX), Taha Masood (Plano, TX), Steven Patterson (Plano, TX), Nuditha Amarasinghe (Richardson, TX), Jerome Butler (Richardson, TX)
Application Number: 11/055,128
International Classification: H01S 3/10 (20060101);